If either of the contributing threads should break upstream of the merger point, the remaining thread would continue for some time to be pulled by the drawing means to produce a defective length of yarn even if the machine were promptly deactivated--manually or automatically--in response to a signal from a sensor detecting the break. It is therefore desirable that a thread guard have means responsive to a rupture of either thread for quickly breaking the other thread in order to prevent the wasting of a significant portion of that thread and to avoid the production of a defective yarn section. Thus, German utility model No. GM 79 12 423 discloses a thread guard of this nature comprising a swingable member disposed just downstream of a thread junction for guiding engagement with the yarn in a normal position of that member from which it is limitedly displaceable to one side or the other by minor differences in the tension of the contributing threads. When this difference exceeds a certain threshold, as will be the case in the event of a thread rupture, the guide member is deflected into an off-normal position in which it impedes the advance of the intact thread so as to cause its rupture. In a specific instance, the guide member is balanced on a horizontal pin and its normal position is metastable so that gravity makes it rotate through 180.degree. when the limits of lateral deflection are surpassed whereby two pins bracketing the yarn invert their relative position to entangle the remaining thread. Also mentioned is the possibility that gravity be replaced by some other stored forces, such as the stress of a spring or an electromagnetic field, to create something like a toggle effect when the guided yarn strongly deviates from its regular path. In any event, the yarn is guided with considerable lateral play by the swingable member in its metastable normal position.
In our copending application Ser. No. 443,561, filed Nov. 22, 1982, now U.S. Pat. No. 4,472,932, we have disclosed an improvement over the subject matter of the above-identified German utility model according to which a carrier rotatable about an axis perpendicular to the path of the yarn and provided with guide means bracketing the yarn in a working position as known from the German publication, is normally immobilized in that working position by detent means deactivable by drive means responsive to detection of a thread break whereupon the carrier is rotated about its axis to invert the position of the guide means. The latter may comprise two pins flanking the yarn or an eyelet traversed by same, as known per se from that publication.
Our copending application, whose disclosure we wish to incorporate by reference into the present one, further describes and shows sensing means separate from the rotatable carrier for monitoring the integrity of the threads upstream of their junction and activating the drive means upon detecting a break. We have found, however, that in some instances the thread guard of the German utility model as well as the sensing means of our copending application may respond in an untimely manner to the lack of normal yarn tension during the stopping or the startup of the machine, thereby blocking the advance of the yarn at the beginning of a new operating cycle. Such an undesirable effect may also result from manipulation of the equipment--e.g. cleaning--during standstill of the machine.